The present invention relates to a method of detecting external impacts in vehicle steering systems which include a rotatable steering shaft and a servomotor for generating a steering assist torque.
External impacts in this connection are intended to mean sudden external influences on the steered wheels of a vehicle as may appear, for example, when driving against a curb at an angle or in the event of unevenness of the road. Such influences may turn out to be so heavy that a steering gear of the vehicle steering system is subjected to extreme mechanical stress. Depending on the magnitude and frequency of such extreme loads, impairments regarding service life and function can not be excluded.
The stress on the steering gear as caused by external impacts is especially pronounced in particular in electromechanical vehicle steering systems since, in comparison with hydraulic or electrohydraulic systems, they exhibit a higher mass inertia and higher system rigidity as well as lower damping.
In order to maintain the safety-relevant steering function of a vehicle over a desired service life, it is common nowadays to overdimension the mechanical components of the vehicle steering system to a certain degree, which, however, adversely affects the overall weight, the space requirement and the costs of the steering system.
DE 103 60 582 A1 in this connection discloses an electromechanical motor vehicle steering system including means for detecting the steering torques applied by the driver, means for detecting the assist torques of an electric servomotor of the steering system and/or means for detecting the steering currents of the electric servomotor. The steering system may also provide means for detecting the system temperature and means for detecting the operating time. In dependence on the values acquired, a control and evaluation device establishes at least one characteristic quantity as at least one degree of damage and generates an output signal or an information output when at least one predefined degree of damage is exceeded. This is to allow a better insight into the condition of stress on or damage to the steering system under the aspect of maintaining high safety and keeping maintenance expenditure low.
In this connection it is a problem that in the occurrence of external impacts, the measured values of the individual characteristic quantities of the vehicle steering system are on the same order of magnitude as the measured values for “normal”, i.e. driver-initiated steering maneuvers. The individual measured values that are due to external impacts are typically even distinctly below the measured maximum values as occur in typical steering movements in vehicle operation. As a result, an identification of external impact events turns out to be fairly difficult.